Electron-transfer pathway for photoinduced Diels-Alder cycloadditions

The photoinduced [4+2] cycloadditions of anthracene to maleic anhydride and various maleimides yield the Diels-Alder adducts in high (chemical) yields. Analysis of the efficiency of these photoreactions as a function of the dienophile concentration leads to limiting quantum efficiencies of Phi=0.014 and Phi=0.16 for maleic anhydride and N-(pentafluorophenyl)maleimide, respectively. Picosecond time-resolved spectroscopy reveals the one-electron transfer from excited (singlet) anthracene to the dienophile acceptor (resulting in the formation of anthracene cation radical and dienophile anion radical) as the critical step prior to cycloaddition. Competition between coupling, back-electron transfer and dissociation of the ion-radical pair which depends on the solvent polarity and the presence of added (inert) salt, limits the quantum yields of adduct formation. (C) 1999 Elsevier Science S.A. All rights reserved.